Recently LANs have taken on added significance in the field of computer systems. Current advancements point to the desirability of interconnecting computers on an organization-wide basis to obtain better sharing of computing capacity. LANs are the means by which computers are typically interconnected on an effective basis for this purpose. However, the data throughput capacities of many LANs must be increased in order to effectively link high capacity devices such as file servers and computational accelerators.
The usual technique for transferring data over a network is to transmit a single digital bit with a single pulse or signal applied to the communication medium, thus transferring one digital bit per signal element. Increasing the data transfer rates by increasing the signaling frequency will generally prove unsuitable because the communication protocol of the network will generally not permit a changed signaling rate, and in many cases, the network communication medium (cables, etc.) will not adequately or reliably support transfer data at higher rates.
Attempts have been made to increase data throughput of a LAN by amplitude modulating the transmitted signals, in order to convey multiple digital bits per signal element. Amplitude modulating the signals results in the signal amplitude, not the presence or the absence of the signal, representing a digital code consisting of multiple bits. For example, by modulating the amplitude of the signal in eight discrete amplitudes or gradations, it is possible to communicate three digital bits of information by each amplitude modulated signal. In this example, amplitude modulation will convey three times as much digital information per signal element compared to the commonly used technique of communicating a single bit per signal element. U.S. Pat. No. 4,602,365 discloses an example of an amplitude modulated LAN. Another example is contained in abandoned U.S. patent application Ser. No. 466,075, filed Feb. 14, 1983.
Attempts at amplitude modulating the signals on a LAN appear to have either not succeeded in actual practice or have met with such limited utility that amplitude modulation techniques for LANs have not achieved widespread commercial use. Perhaps the failures or limited utility of such previous attempts can be explained by the failure to appreciate many of the factors involved in the improvements of the present invention.
Accurate synchronization to the incoming stream of signal elements is required to communicate effectively by amplitude modulating signals over a LAN. Since the amplitude of the received signal represents a digital code, it is necessary to accurately sample the maximum amplitude of the received signal in order to accurately decode its level or amplitude. If the amplitude of the received signal is not accurately sampled at its maximum amplitude because of phase errors in synchronization, inaccurate data decoding will result. Each of the gradations of amplitude in the signal must be reliably detected and distinguished from one another and from noise and other spurious effects on the network medium. Even if the received signal is accurately sampled at its maximum point, the effects on the signal created by the resistance, reactivity, and integrity of the communication medium, by noise, and by amplification created by LAN repeaters, influence the amplitude of each signal when it is received. The residual effects of the previous signal can also influence the amplitude of the next signal. This residual effect, known as intersymbol interference, can effectively change the amplitude of the next signal to an undesired value unless the effects of the intersymbol interference are eliminated from the received signal. The amplitude or gradation difference between different signal levels could theoretically be increased to make the amplitude of each discrete signal easier to discriminate, but there are practical limitations to the maximum signal amplitude which can be conducted by the network medium. It is possible to increase the gradations between each amplitude without unnecessarily increasing the maximum signal levels, but only by decreasing the number of available amplitudes for each signal pulse. Decreasing the available number of acceptable amplitude gradations reduces the amount of data which can be transferred with each signal, which is a result counter to the objective of increasing the amount of data transferred per signal element. Noise, distortion and spurious effects may achieve amplitude levels comparable to the lower levels of the amplitude modulated signals. It is therefore essential to be able to distinguish the lower level signals from the noise and distortion. A more complete discussion of these considerations involved in the improvements available from the present invention is presented below.